Thereby the function of a first sector of a current profile is that the reducing agent metering valve is opened quickly and the function of a second sector is to keep the already opened valve open with a small average amperage in order to control a flow of reducing agent. Such a procedure as well as such a controller are known for an application in motor vehicles like automobiles and trucks from the publication Diesel motor-Management, 4th Edition, Friedrich Vieweg und Sohn Publishers, ISBN 3-528-23873-9, p. 338.
The selective reduction of nitrous oxides (SCR=selective catalytic reduction) is based on a reduction of nitrous oxides (NOx) by selected reducing agents even in the presence of oxygen. Selective means at this, that the oxidation of the reducing agent takes preferably place (selectively) with the oxygen of the nitrous oxides and not with the molecular oxygen that is essentially abundant in the exhaust gas. Ammoniac (NH3) has approved itself as reducing agent with the highest selectiveness. In motor vehicles ammoniac is not carried along in pure form, but dosed to the exhaust gas from a urea-water-solution that has been carried along. Urea (NH2)2CO shows a very good solubility in water and can therefore be simply dosed to the exhaust gas. If it is referred to a reducing agent in this application, this term shall also describe the primary products, the vehicles and media like water, in which a vehicle or the reducing agent is contained in dissolved form. Therefore also the urea-water-solution is described in the following as reducing agent that has to be dosed.
A urea-water-solution with a mass concentration of 32.5% urea, which is known under the trade name AdBlue, has a freezing point at −11 C. A eutectic mixture establishes there, whereby an unmixing of the solution is excluded in the case of a freezing.
Even if an undesired unmixing does not occur at this composition, a freezing of the reducing agent metering valve and other components of the system, for example a freezing of conductions, has to be prevented where possible. At a frozen system the reducing agent could not be dosed anymore, which would cause increased nitrous oxide emissions of the motor vehicle. If the system should still freeze under adverse environmental conditions, it has to be rebuilt in the operation of a motor vehicle. This applies especially to the reducing agent metering valve that generally consists of several metals and synthetics. The reducing agent metering valve is arranged directly at the exhaust gas system. Therefore the danger exists that it is overheated at a hot exhaust system and hot exhaust gas. In order to prevent such a thermal damage, the reducing agent metering valve is generally provided with a cooling element, which allows a deduction of big heat quantity to the environment. In the opposite case of low temperatures the cooling element increases the risk of a freezing of the reducing agent metering valve and aggravates the defrosting of a frozen reducing agent metering valve.